A core drill has conventionally been used for boring a comparatively large hole for passing a water pipe, a gas pipe or a pipe for an air conditioner to a wall, a floor, a foundation or the like constituted by concrete, a stone material or the like in constructing or adding or reforming a building. A core drill is attached with drilling blades formed by sintering diamond abrasive grains by a metal bond at a lower end edge of a core main body formed in a cylindrical shape circumferentially at intervals and by pressing the drilling blades to a concrete face and rotating the core main body, a groove in a shape of a circular ring is cut to form at concrete or the like and by cutting the groove gradually deeply, a hole penetrating concrete or the like is bored. The core drills are used in tools of a wet type cutting the hole by supplying a fluid for cooling to the drilling blades and a dry type which does not supply the cooling fluid.
In drilling operation by the core drill, when the drilling blades formed at a front end thereof cut concrete, a stone material or the like, a large amount of chips of concrete or the like is produced. When the chips are clogged between the core main body and an inner wall face of the bored hole, a resistance against rotating the core drill is increased to thereby reduce a drilling efficiency. Although when the core drill is used in the wet-type tool, the chips are evacuated comparatively efficiently by operation of making the cooling fluid flow, in operation by the dry type tool, the chips may not be evacuated sufficiently to thereby prolong an operational time period of boring.
Therefore, according to a core drill of a background art, an outer peripheral face of a core main body is formed with a chip evacuating groove in a spiral shape and chips produced by drilling blades at a front end thereof are evacuated to an upper side of the core main body via the groove by rotating the core drill (refer to, for example, JP-B-06-092083). Further, there is known a core drill improving evacuation of chips by forming an abrasive grain layer at a surface of a projected streak portion formed at an outer peripheral face of a core main body formed with a groove for evacuating chips and further finely grinding chips produced at drilling blades at a front end thereof by the abrasive grain layer (refer to, for example, JP-A-2000-309013)
According to the core drill of the background art, the chip evacuating groove for evacuating chips is formed in the spiral shape at the outer peripheral face of the core main body and therefore, there poses a problem that production cost is increased by requiring a number of steps in working by, for example, a lathe in producing the core main body. Further, according to the core drill of the background art, the groove for evacuating chips is formed by the same sectional area from a front end side to an upper end of the core drill and therefore, there poses a problem that operation of evacuating chips is not sufficiently carried out, chips formed at the front end portion is compressed to clog at inside of the groove, and the chips are brought into close contact with a wall face of the cut hole of concrete to thereby hamper the drilling efficiency.
Further, according to a hand-held tool for carrying out drilling operation by grabbing the tool by the hand, the outer peripheral face of the core main body is brought into contact with an inner wall face of a concrete hole in a wide area by swinging an axis line of the core drill and therefore, there also poses a problem that a drilling function is reduced by reducing a rotational speed of the core drill by increasing a friction resistance. In order to rotate the core drill at high speed against the friction resistance, a large-sized tool having a larger driving force needs to use.